1. Field of the Invention
Disclosed is a method and design for an apparatus intended for absolute, high-speed measurement of the diameter of transparent fibers and for controlling internal and superficial defects.
2. Description of Related Art
Since 1970, much effort has been devoted to studying the propagation of light perpendicularly to the axis of the fiber. Over the years, this work has yielded many methods and solutions for dimensional measuring of fibers and controlling defects. Among these studies, we referred to:    [1] Saint-Etienne University Thesis, by Lionel Delauney, Mar. 28, 1986    [2] U.S. Pat. No. 3,982,816 by Laurence S. Watkins, Sep. 28, 1976.    [3] U.S. Pat. No. 4,027,977 by Ralph E. Frazee, Jun. 7, 1977    [4] U.S. Pat. No. 4,280,827 by Edward F. Murphy, Jul. 28, 1981.    [5] U.S. Pat. No. 4,176,961 by Ralph E. Frazee, Dec. 4, 1979.    [6] EP 0 549 914 B1, by Leslie James Button, Corning Inc., May 7, 1997 and earlier documents.    [7] 3396052966 FCOTAC, Alcatel FO
The production of telecommunication optical fibers by fibering requires increasingly weak dispersions around the specified nominal diameter and an absence of critical defects affecting the mechanical and optical characteristics of the fibers, but especially those affecting their lifetime. This is the case for “air-line” glass defects, or “bubble” or “delamination” coating defects, as well as the eccentricity of the glass in its polymer coating. Finally, increasing fibering speeds and high-frequency fluctuations of the diameter of the fibers require instruments which are faster that those currently used for visualizing and reducing these phenomena.
References [1] and [2] summarize the fundamental principles of light propagation in a transparent fiber, as described in FIG. 1, specifically when it is illuminated by a monochromatically-collimated light field which is consistent and perpendicular to the axis of the fiber. The deflection of the light by the fiber produces a system of interferometry fringes whose periods and phases depend on the index profiles: diameters, substances, homogeneity, concentricity. By analyzing these fringes according to the measurement angles, one may deduce precise information as to the conformity of the fiber to a model defined as an absolute reference. The quality of the measured signal, represented by the contrast and regularity of the fringes and by the signal's energy, depends on the fiber's optical quality, namely the absence of defects, homogeneity, and geometric regularity. Continuous high-speed analysis of the fringe contrast allows very small defects to be detected, such as bubbles in polymer coatings.
Reference [1] describes in detail the propagation relations and deduces from them a method for rapid measurement of diameter variations and a diameter measurement method by calibration on a standard fiber. The cited documents describe assemblies for measuring diameter variations by counting the fringe shift or the diameter by measuring the reference fringe period to a standard fiber period. Document [6] uses a Fourier transform processing method for diameter measurement and to detect small defects in the glass.
A method and an assembly that allows both dimensional measuring and control of glass fibers prior to coating (bare fibers), as well as dimensional measuring and control of the coatings, would be desirable. More generally, a method and an assembly that applies to any circular section, optically-transparent stem corresponding to FIG. 1 and presenting an R1, N1 and R2, N2 index profile would be desirable. No apparatus known to date allows absolute measurement of the diameter without contact, at more than 50 KHz, and statically with resolutions of a few hundredths of a micron including defect detection.